Linear actuators find widespread applications in industrial, commercial, vehicular, and domestic settings, in uses ranging widely from electric door locks and windshield wipers in automobiles to pin pullers and shutter controllers in mechanical designs. Generally speaking, linear actuators comprise solenoid devices in which an electromagnet is used to translate an armature, and the retraction or extension of the armature is operatively connected in a mechanism to perform useful work. Such devices are commodity items that are manufactured in many sizes, force/stroke outputs, and AC or DC operation.
Despite their widespread adoption, electromagnetic linear actuators have several important drawbacks that require design accommodations in mechanical systems. Due to the use of electromagnetism as the motive force, these devices necessarily require ferromagnetic materials to define the armature as well as a magnetic flux circuit to maximize the stroke force. Such materials are typically dense, and their use results in devices that are rather large and heavy, particularly in comparison to their stroke/force output characteristics. Moreover, the multiple turns of wire that comprise an electromagnet, typically hundreds or thousands, add another substantial mass to the device.
Another drawback of electromagnetic linear actuators is also due to the use of electromagnetism as the driving force. Typically, as the armature is extended from the electromagnetic, increasing portions of the armature are removed from the influence of the electromagnetic field, and the driving force is concomitantly reduced. As a result, the force versus stroke displacement characteristics of these devices generally exhibit high initial force values that decline rapidly with increase in stroke displacement. In many mechanisms it is desirable to deliver a constant force linear stroke, and it is necessary to design additional mechanisms to make use of the negatively sloped force/displacement characteristic.
In recent years much interest has been directed toward shape memory alloy (hereinafter, SMA) materials and their potential use in linear actuators. The most promising material is nickel titanium alloy, known as Nitinol, which, in the form of a wire or bar, delivers a strong contraction force upon heating above a well-defined transition temperature, and which relaxes when cooled. Assuming the Nitinol wire is heated ohmically or by extrinsic means, there is no need for the ferromagnetic materials and numerous windings of the prior art electromagnetic linear actuators, and there is the promise of a lightweight linear actuator that delivers a strong actuation force. Moreover, the force versus displacement characteristic of SMA is much closer to the ideal constant than comparable electromagnetic devices.
Despite the great interest in SMA actuators and many forms of SMA actuators known in the prior art, no practical SMA actuator mechanism has proven to be reliable over a large number of operating cycles. It has been found that Nitinol wire requires a restoring force to assist the material in resuming its quiescent length when its temperature falls below the material""s transition temperature. Many prior art SMA actuator designs have made use of common spring assemblies, such as helical or leaf springs, to exert the required restoring force. These spring assemblies typically deliver a spring force that varies linearly with displacement, (F=kx), and the restoring force in most cases is a maximum at maximum stroke. It has been found that the SMA component responds poorly to this force/displacement characteristic, and the useful life of the SMA actuator is severely limited by such a restoring force. To overcome this problem, prior art designers have attempted to use simple weights depending from pulleys to exert a constant restoring force on the SMA component. Although more effective, this expedient results in a mechanism that is not easily realized in a small, widely adaptive package.
Another drawback inherent in known SMA materials is the relatively small amount of contraction that is exerted upon heating past the transition temperature. The maximum contraction is about 8%, and the useful contraction for repeated use is about 6%. Thus, to achieve a direct displacement stroke from the SMA component of about one inch, the SMA component must be over sixteen inches long. This material limitation results in a minimum size that is too large for many applications. Some prior art designs overcome this problem by wrapping the SMA wire about one or more pulleys to contain the necessary length within a shorter space. However, the SMA wire tends to acquire some of the curvature of the pulleys as it is repeatedly heated and cooled, and loses too much of its ability to contract longitudinally. The result is failure after a few number of operating cycles. Other prior art designs employ lever arrangements or the like to amplify the SMA displacement, with a concomitant reduction in output force.
It is evident that the prior art has failed to fully exploit the full potential of shape memory alloy, due to the lack of a mechanism that capitalizes on the useful material characteristics of SMA.
The present invention generally comprises a linear actuator that employs a shape memory alloy component to deliver a relatively long stroke displacement and reiterative operation over a large number of cycles.
In one aspect, the invention provides a plurality of SMA sub-modules, each capable of displacement upon heating of the respective SMA component. The sub-modules are linked in a serial mechanical connection that combines the stroke displacement of the sub-modules in additive fashion to achieve a relatively long output stroke. Moreover, the sub-modules may be assembled in a small volume, resulting in an actuator of minimal size and maximum stroke displacement.
The sub-modules may be fabricated as rods or bars adapted to be disposed in closely spaced adjacent relationship, each rod or bar linked in serial mechanical connection to the adjacent rod or bar. Alternatively, the sub-modules may comprise concentric motive elements, with the serial mechanical connection extending from each motive element to the radially inwardly adjacent motive element, whereby the innermost motive element receives the sum of the translational excursions of all the motive elements concentric to the innermost element. For all the sub-module embodiments, the serial links therebetween are provided by one or more shape memory alloy wires, each wire connected at opposed ends of adjacent sub-modules to apply contractile force therebetween.
In another aspect, the invention provides an SMA linear actuator assembly employing a spring assembly that is designed to apply a restoring force tailored to optimize the longevity of the SMA component. In one embodiment of the spring assembly, a roller/band spring (hereinafter, rolamite) is connected to the output shaft of the linear actuator assembly. The rolamite spring exerts a restoring force characterized by a decrease in force with increasing displacement, so that the SMA components are returned to their quiescent form with a minimum of residual strain. In a further embodiment, the spring assembly is comprised of a bar or rod connected to the output shaft of the SMA actuator assembly and confined in a channel for longitudinal translation therein. The bar includes shaped cam surfaces extending longitudinally therealong, and a cam follower extends from the channel and is resiliently biased to engage the cam surfaces. As the bar is translated by actuation of the SMA linear actuator assembly, the cam follower exerts a restoring force that is a function of the slope of the cam surface and the magnitude of the resilient force on the cam follower. By appropriate shaping of the cam surface, the assembly exerts on the SMA linear actuator assembly a restoring force characterized by a decrease in force with increasing displacement, whereby the number of cycles of operation is maximized.
In a further aspect, the invention includes a housing in which a plurality of drive rods are arrayed in generally parallel, adjacent relationship and supported to translate freely in their longitudinal directions. One end of each drive rod is connected to the opposed end of an adjacent drive rod by an SMA wire, defining a series of drive assemblies connected in additive, serially linked chain fashion. At one end of the chain, the drive assembly is joined by an SMA wire to the housing, and at the other end of the chain, the housing is provided with an opening through which an actuating rod may extend. Also secured in the housing is a spring, such as a rolamite roller/band spring, having one end connected to the housing and the other end connected to the actuator rod. The spring is designed to exert a restoring force having a constant or negative force versus displacement relationship.
Each SMA wire is connected in an electrical circuit, in one of several arrangements of series or parallel connections, so that ohmic heating may be employed to heat the SMA wires beyond their phase transition temperature. In the chain-connected series of SMA drive assemblies, the resulting contraction of the SMA wires is cumulative and additive, and the actuating rod is driven to extend from the housing with a high force output. When the current in the circuit is terminated, the SMA wires cool below the transition temperature, and the spring restores the SMA wires to their quiescent length by urging the actuating rod to translate retrograde and (through the chained connection of assemblies) to apply sufficient tension to re-extend all the SMA wires.
It may be appreciated that the SMA wires remain in substantially linear dispositions throughout the contraction/extension cycle, so that flex-induced stresses are avoided. To assist in heat removal for high power applications, the housing may be filled with oil or other thermal absorber, which may be cooled passively or actively. To deliver additional force, two or more SMA wires may be connected between the drive assemblies, rather than one wire. To provide enhanced actuation and retraction times, the SMA wires may be thinner.
Although the invention is described with reference to the shape memory component comprising a wire formed of Nitinol, it is intended to encompass any shape memory material in any form that is consonant with the structure and concept of the invention.